Traditionally, downhole and crosshole seismic testing methods have been used to obtain numerous in-situ measurements of various geotechnical properties. For example, downhole and crosshole seismic testing methods have been used to measure compression wave (also known as P-wave) and shear wave (also known as S-wave) velocity profiles (Vp and Vs, respectively) as well as to measure dynamic soil/rock properties, to identify soil stratification and to determine shear modulus.
The P-wave and S-wave velocity profiles measured by downhole and crosshole seismic testing have multiple uses in geotechnical design and analysis, such as the prediction and evaluation of site response to earthquake shaking. Sites often either amplify or attenuate earthquakes based upon the spectral content of the earthquake and the resonant properties of the soil/rock column. Borehole seismic data may also be used for estimating soil strength, rock rippability (i.e., the ease of excavating or blasting the rock for construction purposes) and liquefaction potential (i.e., the susceptibility of the soil to liquefaction during an earthquake) as well as void detection and two-dimensional tomography.
Generally, downhole and crosshole seismic testing methods have employed the use of multi-component (i.e., two- or three-component) borehole receiving devices. Specifically, these devices include one or more receivers or geophones enclosed within a cylindrical housing made of metal or plastic. The housing is lowered into a cased or uncased borehole. In order to attempt to achieve intimate coupling between the geophones and the borehole wall, which is advantageous for obtaining accurate measurements of the P-wave and S-wave velocity profiles, an inflatable bladder (separate and distinct from the geophones housing) is used in conjunction with the geophones housing. A traditional borehole receiving device 10 with an inflatable bladder 20 and a receiver housing 30 is illustrated in FIG. 1. Alternatively, a spring-loaded or clamping mechanism may be used. In use, the bladder or other mechanism is lowered into the borehole and inflated in order to attempt to achieve intimate coupling between the geophones 40 and the borehole wall 50.
Although the conventional configuration for borehole receiving devices discussed above has been used for decades, these multi-component units suffer from certain limitations. As illustrated in FIG. 2, when the borehole wall is irregularly shaped, it is difficult to achieve intimate coupling between the geophones housed within the casing or housing and the borehole wall due to the use of the separate inflatable bladder. As a result, accurate P-wave and S-wave velocity profiles measurements are difficult (if not impossible) to achieve.
Even when intimate coupling is possible between the geophones and the borehole wall (i.e., when the borehole wall is regularly shaped) such that accurate P-wave and S-wave velocity profile measurements may be ascertained, the receiving devices are still relatively heavy, cumbersome and difficult to use. Consequently, these devices are difficult to transport to and from the desired borehole site. In particular, these devices often weigh at least several kilograms and may be up to half a meter (0.5 meters or 50 centimeters) in length.
Accordingly, the art of borehole receivers for seismic testing has a need for a borehole receiver that incorporates the housing and inflatable bladder into a single, compact unit to ensure excellent coupling between the device and the borehole wall regardless of the shape of the borehole wall. In addition, the art of borehole receivers for seismic testing has a need for a simpler, lightweight borehole receiver for seismic testing. Finally, the art of borehole receivers for seismic testing has a need for a borehole receiver that is easily constructed and having dimensions that are easily modified to suit any borehole diameter.